MS16.3 - Recent advances in vibration control of structures with isolation and/or energy dissipation devices

Engineering structures are known to be susceptible to the adverse effects of dynamic vibration, which can affect serviceability and/or integrity, resulting to limited or unsafe operation and damage. The mitigation of the effects of vibration excitation sources can be accomplished via external devices, which are attached to the primary system as a means for absorbing energy. The further alteration of the original system’s natural frequency or damping characteristics can lead to reduction of the experienced vibrations. In rendering vibration mitigation devices efficient, these need to mitigate dynamic vibration under broad set of inputs and for a wide range of frequencies. As a particular source of interest, seismic excitation is typically characterized by low frequency contents, resulting in relatively large displacement amplitudes. The design of such devices is further dictated by limitations on feasibility and ease of deployment, which affects the practicality of use on real structures. One potential solution is the protection of structures with the inclusion of nonlinear elements that can offer variable stiffness properties depending on the displacement amplitude. The current study investigates a geometrically nonlinear device for vibration mitigation. A bi-stable element is used to generate negative stiffness effects and is utilized for limiting structural response amplitudes. It is shown that a shift in the stiffness characteristics of a multi-storey frame can be achieved, thus modifying its effective dynamic properties. The input energy of the dynamic excitation can be channelled into and consumed at specific locations of the system. Crucial elements of the structure can therefore be protected, limiting damage and reducing capacity requirements. Analytical calculations are performed on a lumped mass model to identify the frequency response of the modified structure in comparison to the original, while they provide specifications and limitations for the tuning parameters of the device. Furthermore, numerical analyses are performed for sinusoidal inputs, which are in agreement with analytical calculations, while the response under realistic seismic records is additionally studied. The behaviour of the device is explored for varying geometric, stiffness and damping properties to identify the range of parameters, where the system is effective and vibration mitigation is achieved. For the evaluation of the system’s performance, several criteria are used, including energy based measures, as well as the acceleration response. The results clearly indicate the ability of the device to mitigate vibration for a protected structure and alleviate energy accumulation at crucial locations. The proposed mechanism is capable of offering protection for 2-dimensional horizontal excitation, thus being useful for practical applications. The effectiveness of the modified system is demonstrated for a purely harmonic, as well as a seismic input, showing potential for the protection of real structures.

The high demands of renewable energy have instigated the increase of wind farms’ capacity, which have required larger wind turbines and, consequently, monopile foundations of larger dimensions. The installation of offshore monopiles, which is typically performed by an impact hammer, generates high noise levels that can cause severe damage to marine wildlife, such as hearing injury, behavioral disturbance, or even death. Although current noise attenuation techniques used in this process have shown a significant noise reduction at high frequency ranges, mitigating low-frequency noise is still extremely challenging and becomes a concern for the new wind turbine models. To address such problem, here we propose an elastic metamaterial-based structure composed of single-phase resonant structures. The proposed structure, which we call a meta-interface, is introduced between the monopile and the hammer and is used to remove energy from the input signal associated with high noise levels. To that end, we first identify the frequency ranges associated with high sound pressure levels, which were shown to be related to the monopile’s eigenmodes. Then we design the meta-interface’s periodic unit cells so that the elastic/acoustic waves at identified frequency ranges are attenuated. A meta-interface is then realized by replicating the unit cell along the monopile wall (matching the thickness) to form a ring-shaped layer, and then by stacking up these concentric layers. A frequency analysis of the pile driving system with the meta-interface shows that the new noise levels attain a significant attenuation in frequency ranges lower than 1000 Hz.

Wind Turbines (WT) are rapidly becoming a significant contributor of the total energy produc-tion grid. Nowadays, a typical WT has the capability of 15MW power generation with larger rotors and towers with the average tower height being 120m. These slender and flexible struc-tures are vulnerable to external vibration sources such as wind gusts and earthquake excita-tions. To ensure safety and sustainability, it is necessary to mitigate the dynamic responses of the WT. The integration of dynamic vibration absorbers (DVA) to WT towers has the potential to significantly improve the damping of the tower and the nacelle dynamic responses, increas-ing thus the reliability of WTs. Current research is focused in studying the dynamic response of WT with DVA being installed at the top of the tower. In that case, DVA is indeed effective in suppressing the fundamental vibration mode of the tower, in which the maximum displacement occurs at the nacelle. However, when the WT is subjected to earthquake excitation, higher vi-bration modes are also excited. These modes may further contribute or even dominate the dy-namics of the tower. In this study, the structural response of WT with DVA installed at various positions along the tower height, is presented. Results indicate that in order to effectively con-trol the tower vibrations, two DVA need to be installed, one below the nacelle and one at the base of the tower. Further investigation is being performed to identify the optimum location and design of these DVAs. Finally, it is shown that vibration control can extend the lifetime of the structure increasing the WTs reliability.

In important facilities such as nuclear power plants or plant facility data centers, the safety of both the structure as well as the facilities and equipment installed in the structure must be ensured. This is because when the equipment installed in the structure is damaged, nuclear power plants can cause fatal damages, such as core damage or severe social damage caused by failure to perform the main functions of the facility. To reduce the intensity of the seismic response of the installed equipment effectively, the entire structure should be used as a seismic isolation structure. In this context, the floor response spectrum (FRS) must be analyzed to evaluate the seismic response on the equipment installed on each floor. The FRS has a peak value at the frequency corresponding to the structural vibration mode, but the frequency and amplitude of the peak can vary because of several uncertainties such as site characteristics, soil structure interaction, and structural vibration characteristics. Therefore, the broadening of the peak frequency in FRS has been introduced in order to evaluate the design FRS conservatively. The criteria for FRS peak broadening for fixed base structures was suggested, but there are no suggested criteria for seismically isolated structures. In terms of seismic isolation structures, whether the criteria of a fixed structure can be applied must be verified, since they are obtained via analysis, the isolator’s initial stiffness, and the uncertainty of ground motion due to nonlinearity. In this study, the vibration of FRS due to the initial stiffness of bi linear behavior of isolators and the intensity and duration of ground motions was evaluated that have not been reviewed in detail when designing isolators and generating FRS of isolated structures. By analyzing a simplified structural model for base isolated structure, it appears that the natural period of the fixed base structure has shifted owing to the seismic isolation system, and this phenomenon was confirmed by solving the equation of motion of the two degree-of-freedom seismic isolation system. It was found that the uncertainty of initial stiffness of isolators also affects significantly to the FRS. The FRS of earthquake waves weaker than the design earthquake intensity was almost overlapped by that of the existing design earthquake intensity. Also, the FRS changed significantly depending on the strong motion duration of the earthquake, and the shorter the strong motion duration of the earthquake, the higher was the peak value only in certain periods, such as the seismic isolation period. In the case of seismic isolation structures, the FRS increases nonlinearly as the earthquake intensity increases from a low earthquake intensity owing to the nonlinear behavior of the isolator. This phenomenon is particularly important when evaluating the seismic intensity of equipment exceeding the design earthquake intensity, and it can be applied by expanding the characteristics of the FRS based on the earthquake intensity discussed herein. From these results, the several considerations for generating design FRS for seismically isolated structures were suggested.